Citation or identification of any reference in Section 2 of this application shall not be construed as an admission that such reference is available as prior art to the present invention.
A major problem in the chemotherapy of solid tumor cancers is the delivery of therapeutic agents, such as drugs, in sufficient concentrations to eradicate tumor cells while at the same time minimizing damage to normal cells. Thus, studies in many laboratories are directed toward the design of biological delivery systems, such as antibodies, cytokines, and viruses for targeted delivery of drugs, pro-drug converting enzymes, and/or genes into tumor cells. Houghton and Colt, 1993, New Perspectives in Cancer Diagnosis and Management 1: 65-70; de Palazzo, et al., 1992a, Cell. Immunol. 142:338-347; de Palazzo et al., 1992b, Cancer Res. 52: 5713-5719; Weiner, et al., 1993a, J. Immunotherapy 13:110-116; Weiner et al., 1993b, J. Immunol. 151:2877-2886; Adams et al., 1993, Cancer Res. 53:4026-4034; Fanger et al., 1990, FASEB J. 4:2846-2849; Fanger et al., 1991, Immunol. Today 12:51-54; Segal, et al., 1991, Ann N.Y. Acad. Sci. 636:288-294; Segal et al., 1992, Immunobiology 185:390-402; Wunderlich et al., 1992; Intl. J. Clin. Lab. Res. 22:17-20; George et al., 1994, J. Immunol. 152:1802-1811; Huston et al., 1993, Intl. Rev. Immunol. 10:195-217; Stafford et al., 1993, Cancer Res. 53:4026-4034; Haber et al., 1992, Ann. N.Y. Acad. Sci. 667:365-381; Haber, 1992, Ann. N.Y. Acad. Sci. 667: 365-381; Feloner and Rhodes, 1991, Nature 349:351-352; Sarver and Rossi, 1993, AIDS Research & Human Retroviruses 9:483-487; Levine and Friedmann, 1993, Am. J. Dis. Child 147:1167-1176; Friedmann, 1993, Mol. Genetic Med. 3:1-32; Gilboa and Smith, 1994, Trends in Genetics 10:139-144; Saito et al., 1994, Cancer Res. 54:3516-3520; Li et al., 1994, Blood 83:3403-3408; Vieweg et al., 1994, Cancer Res. 54:1760-1765; Lin et al., 1994, Science 265:666-669; Lu et al., 1994, Human Gene Therapy 5:203-208; Gansbacher et al., 1992, Blood 80:2817-2825; Gastl et al., 1992, Cancer Res. 52:6229-6236.
Because of their biospecificity, such systems could in theory deliver therapeutic agents to tumors. However, it has become apparent that numerous barriers exist in the delivery of therapeutic agents to solid tumors that may compromise the effectiveness of antibodies, cytokines, and viruses as delivery systems. Jain, 1994, Scientific American 7:58-65 (Jain). For example, in order for chemotherapeutic agents to eradicate metastatic tumor cells, they must
a) travel to the tumors via the vasculature; PA1 b) extravasate from the small blood vessels supplying the tumor; PA1 c) traverse through the tumor matrix to reach those tumor cells distal to the blood supply; and PA1 d) interact effectively with the target tumor cells (adherence, invasion, pro-drug activation, etc). PA1 An agent that destroys cancers cells in a culture dish should, in theory, be able to kill such cells in the body . . . . Sadly, however, the existing pharmacopoeia has not markedly reduced the number of deaths caused by the most common solid tumors in adults, among them cancers of the lung, breast, colon, rectum, prostate, and brain . . . . Before a blood-borne drug can begin to attack malignant cells in a tumor, it must accomplish three critical tasks. It has to make its way into a microscopic blood vessel lying near malignant cells in the tumor, exit from the vessel into the surrounding matrix (the interstitium), and finally, migrate through the matrix to the cells. Unfortunately, tumors often develop in ways that hinder each of these steps. PA1 The treatment of cancer by injections of bacterial products is based on the fact that for over two hundred years neoplasms have been observed to regress following acute infections, principally streptococcal. If these cases were not too far advanced and the infections were of sufficient severity or duration, the tumors completely disappeared and the patients remained free from recurrence. PA1 Are pathogenic and non-pathogenic organisms one of Nature's controls of microscopic foci of malignant disease, and in making progress in the control of infectious diseases, are we removing one of Nature's controls of cancer? PA1 The feasibility of utilizing the tumorcidal properties of pathogenic/free-living amoebae and their cell-free products in the treatment of drug-resistant or radio-resistant tumors warrants further investigation.
Although antibodies and viruses can express specific recognition sites for tumor cells, they are dependent solely upon the forces of diffusion and convection in order to reach these sites. According to Jain:
Jain points out that blood vessels supplying tumors are irregular and convoluted in shape so that blood flow is frequently restricted compared to that in normally vascularized tissue. In addition, there is an unusually high interstitial pressure in many tumors that counteracts the blood flow. Jain further points out that the two chief forces governing the transport of agents to tumor cells via the circulatory system are convection (the transport of molecules by a stream of flowing fluid), and diffusion (the movement of molecules from an area of high concentration to an area of low concentration). Since tumors are often non-uniformly vascularized, many cells in the tumors receive nutrients through the process of diffusion through the matrix. Jain and coworkers obtained data suggesting that "a continuously supplied monoclonal antibody having a molecular weight of 150,000 daltons could take several months to reach a uniform concentration in a tumor that measured one centimeter in radius and had no blood supply in its center."